You haven't closed the opening braces either one of your if statements, or the loop function. For every opening curly brace, '{', you MUST have a closing brace, '}'.
int ENA = 5; int IN1 = 4; int IN2 = 3; int ENB = 6; int IN3 = 7; int IN4 = 8;
void setup()
{
pinMode(ENA, OUTPUT);
pinMode(IN1, OUTPUT);
pinMode(IN2, OUTPUT);
pinMode(ENB, OUTPUT);
pinMode(IN3, OUTPUT);
pinMode(IN4, OUTPUT);
Serial.begin(9600); }
void loop()
{
if (Serial.available() > 0)
{
int data = Serial.read();
if (data == 'F')
{ digitalWrite(IN1, HIGH);
digitalWrite(IN2, LOW);
digitalWrite(IN3, HIGH);
digitalWrite(IN4, LOW);
}
}
}